Twenty-five toilets decorated by New South Wales artists have been put on display in Sydney for World Toilet Day - a UN initiative to raise awareness about the number of people around the world without access to toilets.

The installation was organised by non-profit organisations Engineers Without Borders and WaterAid, which are working in developing countries to help construct sanitation systems.

The UN estimates 2.5 billion people around the world do not have access to toilets and proper sanitation facilities.

Mexican tourist Diana Nateras was among the passers-by stopping to examine the unusual exhibition at Circular Quay.

"We were coming back here to our hotel and it's really attractive, so you want to stop," she said.

"Then you get talking to the people here and they tell you about these problems and then you feel like you should donate."